Vegetative Dictyostelium cells containing 17 actin genes express a single major actin

Abstract

Although actin is highly conserved between different eukaryotic species^1–3, six tissue-specific actins have been characterised in higher vertebrates by complete amino acid sequence analysis (two cytoplasmic actins, two smooth muscle actins and two sarcomeric actins)^4–8. Their tissue specificity suggests they may differ in some important although unknown physiological property. Actin expression in lower eukaryotes seems to be a simpler process than in higher eukaryotes since biochemical experiments have indicated only one major type in purified preparations from various species^1–3,9–12. However, Firtel et al. have isolated several recombinant plasmids containing sequences of Dictyostelium discoideum DNA complementary to actin messenger RNA^13–15 and have suggested that this unicellular slime mould may have 17 actin genes^15,16 potentially giving rise to several different actins. We have, therefore, determined the complete amino acid sequence of actin from vegetative Dictyostelium cells. This sequence is unique and agrees with the DNA sequences of four actin genes for that region of the DNA, which is currently known. The protein sequence does not agree with the three other ‘genes’ and we discuss the possible expression of minor actin species.